Signal ratio alarm system



Jan. i6, 1968 N. G. LONG SIGNAL RATIO ALARM SYSTEM 2 Sheets-Sheet l Filed Dec. 18, 1963 MQQEQQM @mi A TOR/v5 Y Jam.. 6, 196% N. cs. LONG SGhhekL RATIO ALARM SYSTEM Filed Dec. 1S, 1953 hmmm@ United States Patent O 3,364,469 SiC-NAL BATH@ ALARM SYSTEM Norwood G. Long, Millington, NJ., assigner to Bell Telephone Laboratories, incorporated, New York, NSY., a corporation ot New York Filed Bec. 13, i963, Ser. No. 331,437 7 Claims. (Cl. 340-449) This invention relates to a method of and apparatus for measuring the ratio of time durations of two independent signals and, more particularly, to a method and apparatus for monitoring the ratio of time durations of two independent signals to determine whether a particular ratio has been attained.

ln many communication systems wherein a plurality of intelligence signals are transmitted on a lesser number of transmission channels, it is desirable to examine the fraction of time that the average signal source is active to provide an adequate number of transmission channels. An important ligure of merit respecting the operation ot such a system is the ratio ot the duration of time for which no channel is available when required to the total duration of activity of the signal sources.

In known systems to measure the ratio of time durations of two independent signals, the use of two independent counters has been necessary to measure the ratio. Each signal to be measured is converted to a pulse train, the number of pulses being proportional to the duration of the signal.

The pulse train representing the denominator of the ratio to be measured is applied to a preset counter. This preset counter is designed to emit a signal following the receipt of a selected number of pulses. The pulse train representing the numerator of the ratio is applied to a second counter. The second pulse train is applied to the second counter through a disabling gate which is actuated by the designated signal pulse output of the preset counter. After disablement of the gate, the ratio of the two signals is represented by the ratio of count attained by the second counter with the selected count number assigned to the preset counter. The critical ratio is monitored by setting a particular threshold alarm ratio corresponding to a preselected count of the second counter.

The above-described critical ratio monitoring system has several inherent disadvantages. The chief disadvantage is that the average time period over which the ratio is determined is not a constant but is completely dependent upon the duration of one ot the .signals and is independent of the other. This means that a critical ratio could possibly be exceeded for a considerable time before one of the signals was of surlicient duration to enable the alarm. Another disadvantage is that two counters are necessary to determine one threshold alarm count. A third disadvantage is that, as the ratio to be checked is altered, the pulse output of the second counter necessary to attain the alarm threshold changes. ribis means that certain ratios will require much more time to attain the same threshold count than others, seriously limiting the time selection of periods over which the ratio can be monitored.

It is therefore an object of the present invention to monitor the time duration ratio of two independent signals for any preselected period of time independent of the ratio to be monitored.

It is another object of the present invention to monitor the time duration ratio of two independent signals with a simpler method than hitherto used, requiring only one counter.

in accordance with the present invention, the two independent signals, whose duration ratio is to be monitored, are used to control the outputs of two pulse gen- ICC erators. The ratio of the repetition rates of the two generators is inversely proportional to the value of the critical signal ratio to be monitored. The one independent signal is used to enable the output of one pulse generator. The other independent signal is used to inhibit the output of the other pulse generator. A counter is provided to record the total number of pulses transmitted from both generators. An alarm indicates the attainment of a predetermined count, representing the attainment of the critical ratio.

These and other objects, the nature of the present invention and its various advantages, will appear more fully upon consideration of the attached drawings and of the following detailed description of the drawings wherein:

FIG. l is a schematic diagram of one illustrative embodiment utilizing the ratio monitoring method of the present invention to monitor the ratio of time durations of two signals; and

FIG. 2 is a schematic diagram of an alternative embodiment of the present invention designed to monitor the critical ratio of the number of two sets of discrete physical objects passing particular reference points.

Referring more particularly to FIG. 1, an illustrative embodiment of the invention is shown which may be used to .monitor ratios of the respective time durations of two separate, intermittent signals. A first signal source 13 of intermittent signals is connected to the input l2 of the coincidence gate 3d. A second signal source 23 of intermittent signals is connected to the input 2i of the anticoincidence gate 40. The signal sources 13 and 23 may comprise any signal sources Whose ratio of durations it is desirable to monitor.

A particular illustrative example of such signal generating means may be found in a communication system, where it is desirable to monitor the ratio of two separate communication signals. Such a system is described in A. R. Kolding Patent 2,957,946, issued Oct. 25, 1960. Described therein is a Time Assignment Speech Interpolation (TASI) system that permits a particular number of parties to speak with each other through a lesser number of communication channels. Basically, the communication system therein described performs its function by assigning a party to an idler communication channel during the time he is speaking. When the party is silent, he is disconnected from the channel so that the speech of another active party may be interpolated on the same channel between the active speech segments of the first party.

lf more parties seek access to the communication channels than there are channels available, part of the speech of some of the parties is lost (frozen out) because there is no available communication channel.

To determine the number of channels necessary to handle a certain number or parties, the ratio of the total activity time ot' the parties speech to the time it is frozen out must be monitored. it is assumed that the signal source 23 generates signals proportional in duration to the parties speech. Similarly, the signal source i3 generates signals proportional to the reezeout time.

A lirst pulse generator lll, having a predetermined characteristic output repetition rate, is connected, via lead 1l, to the coincidence gate 39. A second pulse generator 2h, also having a predetermined but different characteristic output repetition rate, is connected, via lead 22, to the anti-coincidence gate dll. The two pulse generators 10 and 20 may comprise blocking oscillators or any other means for producing pulse trains at predetermined output repetition rates.

the pulse generator 1t) to the repetition rate of the pulse generator 20 is equal to the inver-se of the critical value of the ratio of the duration of the signal output of the signal source 13 to the duration of the signal output of the signal source 23.

The output lead 35 from coincidence gate 30 is connected to an isolating amplifier 6i)l and, via lead 36, to a coincidence gate 50. The coincidence gate 30 may comprise any circuit arrangement that will permit the transmission of a pulse train applied to one of its input leads when the other of its input leads is energized.

The output lead 4S of the anti-coincidence gate di) is connected to an isolating ampliiier 61 and, via lead 46, to a coincidence gate 5t). The anti-coincidence gate 4t) may comprise any circuit arrangement that `Nill prevent the transmission of a pulse train applied to one of its input leads when the other of its input leads is energized. Coincidence and anti-coincidence circuits are well known in the art and will not be further described here.

The output -lead 51 of the coincidence gate Si) is connected, via a delay line S5, to the isolating amplifier 62. The coincidence gate Sii may be identical to the coincidence gate 30. The delay line 55 is designed to delay a pulse signal transmitted through it a length of time at least equal to the pulse width of the pulses from sources and 2t?. Y

The outputs of the three isolating amplifiers 6l), 61, and 62 are connected, via lead 73, to a pulse counting circuit 74). The pulse counting circuit '70 may comprise any type of counting circuit which provides a distinctive output when a particular count is reached.

The output lead 77 of the counter 70 is connected to a detector circuit Sii. The detector circuit Sil may comprise a blocking oscillator or some other circuit responsive to the distinctive output from counter 79.

The output of the detector 80 may be used to operate an alarm or other control or indicating circuit, and is also applied via lead 89 to reset pulse generator 9i). The reset pulse generator 90 is connected, via the reset input lead 91, to the counter 70. The reset pulse generator 9i) may be any circuit arrangement designed to generate a reset pulse whenever triggered by a signal on input lead 89, and at regular intervals thereafter.

Briefly, the operation of the circuit is as follows:

The two separate signals from the signal sources 13 and 23 are applied, respectively, to the input 11 of the coincidence gate 3@ and the input 21 of the anti-coinci-V dence gate 4'0. A pulse train generated by the pulse generator 1t) at a preselected repetition rate is applied to the input 12 of the coincidence gate 3d. During the period that the signal source 13 is active, the coincidence gate 30 is enabled by signals on input 12 to transmit this pulse train to output lead 35, the isolating amplifier 6i), and the pulse counter '70.

A pulse train generated by the pulse generator at some other preselected repetition rate is applied to the input 22 of the anti-coincidence gate 40. The ratio of the repetition rate of pulse generator 20 and the repetition rate of pulse generator 10 is selected, as indicated above, to be equal Vto some preselected value.

During the period that the signal source 23 is active, the anti-coincidence gate 40 prevents the transmission of the pulse train on input Z2 to the output ylead 45. During the period that the signal source 23 is inactive, the pulse train on input 22 is transmitted to output lead 45, the isolating amplifier 61, and the pulse counting circuit 70.

The pulse trains on leads 3S and 45 are also applied,

via leads 35 and 46, respectively, to the coincidence gate Y 50.. The coincidence of pulses of the two respective pulse trams applied thereto permits the transmission of a pulse to the output lead 51. This pulse output is applied, via

the delay line V55 and the isolating amplifier 62, to the pulse counting circuit 70. In this way, the loss of a pulse count when individual pulses of the pulse train on leads 35 and 4S coincide is prevented. The delay line 55 clays the pulse transmitted by the coincidence gate 50 a suicient length of time to prevent overlap w1th the coincident pulses. Y

In summation, if signal source 13 is active, the pulse train output of the pulse generator 10 is transmitted to the pulse counter 7i). Similarly, if the signal source 23 is inactive, the pulse train output of the pulse generator 2t) is transmitted to the pulse counter 7d. The attainment of a preselected count in the pulse counter 7&3y within some predetermined period of time indicates that some preselected ratio being monitored has been attained or exceeded.

After the predetermined time for monitoring has passed, or after the detector has been activated, the counter is reset by generator 90 to initiate the monitoring of the critical ratio for a subsequent time period. The predetermined period of time for which the ratio is mom* tored is `selected by adjusting the time period of the reset pulse generator 9i) to generate a reset pulse at the end of the selected time period. Should the ratio be attained before the time period has expired, the output signal of the detector circuit titi, applied to the generator 9i? via -lead 89, immediately resets the counter circuit.

The operation of the above-described monitoring method may be briefly analyzed as follows:

S is the number of pulses transmitted to counter 70;

A is the number of pulses corresponding to the alarm limit; v

Fa is the pulse repetition rate of the pulse generator 10;

Fb is the pulse repetition rate of the pulse generator 20;

Ta is the time duration of the signal applied to the gate input 12 by the signal source 13;

Tb is the time duration of the signal applied to the gate input 21 by the signal source 23;

R is the critical ratio to be detected where R=Fb/Fa; and

T is a preselected time period over which the ratio is to be checked.

The total number of pulses S transmitted to the counter "75 for the fixed measuring period T may be expressed i and Y By substituting Equations 2 into l we obtain Ithe number of pulses A at which detectormust respond as As Equations 2 and 3 indicate, the present monitoring method requires that the ratio of the frequency. of the two pulse generators must be set inversely proportional td the critical signal duration ratio to be monitored. The number of pulses necessary to indicate the attainmentof this ratio is equal to the quantity of (TFb) pulses and is independent of the time Tb. The adjustment of the repetition rate Fb permits selection of any suitable time duration during which the ratio is monitored.

Referring more particularly to FIG. 2, an embodiment of the invention is shown for the purpose of monitoring the ratio of the number of physical objects traveling on two separate paths. A useful application of this embodiment would be at a statisticalV quality inspection station situated at some point in a manufacturing process'to compare the ratio of the number of parts rejected by the inspection processl to the total number of parts inspected.

Statistical quality inspection is used as a substitute for total inspection of all parts in many modern industrial processes because `of the expense involved in inspecting all parts individually. Generally, out of each commercial lot of goods to be investigated a smaller number of parts is selected at random to form a sublot. All parts of the sublot are inspected. Statistical quality inspection dictates that an entire lot size of goods 'be rejected if a certain threshold percentage of a sublot size of units is found to be defective.

Normally the inspector must resort to computation and graph paper to ascertain if this threshold percentage has been attained. A particularly effective method of measuring this threshold percentage is to measure the ratio of the units rejected to the total number of units inspected in the sublot size. An illustrative embodiment of the present invention, as subsequently presented, can monitor such a ratio.

A single pulse generator 110 is connected, via leads 119 and 129, to a coincidence gate 130 and an anti-coincidence gate 1MB, respectively. These gates may be identical to the gates 3G and 4t] shown in the illustrative embodiment of FIG. l. The discrete objects that have been rejected by the inspector at some inspection station pass through a sensor 112, on the conveyor path 111. The sensor 112 may comprise any device which will detect objects passing by on the conveyor and emit a pulse signal output for each object so detected.

The sensor 112 is connected, via lead 115, to the input of a monostable multivibrator 117. The monostable multivibrator 117 is designed to respond to a signal applied to its input by switching to its unstable state for a specific predetermined time period. The unstable output lterminal of the multivibrator 117 is connected, via lead 118, to the input of the coincidence gate 131D.

The total number of discrete objects of the sublot to be inspected pass through the sensor 122, on the conveyor 121. The sensor 122 is identical in function to the sensor 112. The sensor 122 is connected, via lead 125, to the input of the monostable multivibrator 127. The

monostable multivibrator 127 is designed to respond to a signal applied to its input yby switching to its unstable state for a specific predetermined time period.

The ratio of the respective durations of the unstable states of the two monostable multivibrators 117 and 127 is selected to represent some critical value. This preselected ratio represents the inverse of the critical ratio of the rejected units on conveyor 111 to the total number of units of the sublot inspected on the conveyor 121.

The output of the coincidence gate 130 is connected, via lead 131, to a delay line 137. This delay line 137 is designed to delay a pulse passing through it 'for a period of time at least as great as the time duration of that pulse.I The pulse is delayed to prevent possible coincidence with pulses transmitted by the anti-coincidence gate 140.

The delay line 137 is connected, via the isolating amplitier 160, to the pulse counter 171i. The pulse counter 170 may be identical to the pulse counter 711 used in the illustrative embodiment of FlG. 1. The output of the pulse counter 170 is connected to a detector circuit 180. The detector circuit 1110 is designed to be triggered by a certain threshold count signal attained by the counter 170, which signal indicates that a particular critical ratio has been attained or exceeded.

A reset pulse generator 190 is connected to the reset input 191 of the counting circuit 170. The generator 190 may be identical to the reset pulse generator 90 shown in FlG. l. The output of the detector circuit 180 is connected, via lead 189, to the reset pulse generator 196.

The operation of the above-described embodiment is as follows:

An object passing the sensor 112, on the conveyor 111, causes the sensor 112 to apply a signal, via lead 11S, to the input of the monostable multi-vibrator 117. The monostable multivibrator 117, so activated, switches to its unstable state. During the period of its unstable output state, the multivibrator 117 applies a signal, via lead 118, to the coincidence gate 131i. This signal enables the coincidence gate to transmit the pulse train from pulse generator 110 to the output lead 131.

Similarly, objects passing the sensor 122, on conveyor 121, cause the sensor 122 to apply a signal, via lead 125, to the input of the monostable multivibrator 127. The monostable multivibrator 127 switches to its unstable state and applies a signal, for the duration of that state, to the input lead 128 of the anti-coincidence gate 141i. The input lead 128 is connected to the inhibiting terminal o-f the anti-coincidence gate 140. During the period that the signal is applied to input lead 128, the anti-coincidence gate 141i is prevented from transmitting the pulse train output of the pulse generator 110 to the output lead 141.

The transmitted pulse trains on lea-ds 131 and 141 are applied, via the isolating amplifiers and 161, respectively, to the pulse counter 171). The pulse train on lead 131 is delayed by the delay line 137 to prevent coincidence with the pulse train on lead 141.*The total number of pulses in both pulse trains is summed by the pulse counter 170. The attainment of a certain sum within a preselected time period indicates that a particular ratio has been attained or exceeded.

In summation, every object on conveyor 111 passing the sensor 112 permits a preselected number of pulses to be applied to the pulse counter 170. Every object on conveyor 121 passing the sensor 122 subtracts a preselected number of pulses from a continuous train of pulses directed to the pulse counter 170. The attainment of a certain sum in the pulse counter 171) activates the detector circuit 180. The ratio of the time duration of the unstable state of the two monostable multivibrators 17 and 27 determines the particular critical ratio to be monitored. In the preceding illustrative embodiment the critical ratio was determined by the respective ratio of the pulse repetition rates of the two generators 1i) and 211.

The detector circuit ld, when activated, applies a signal, via lead 189, to reset pulse generator 19t). Counter 17d is thereby reset, enabling the system to continue monitoring the ratio for a subsequent time period. If the ratio has not been attained within the time period of the reset pulse generator 19th, the regular pulse output from that generator is applied, via the reset input 191, to the counter 170.

It is to be understood that the method of the present invention is not limited to the enumerated illustrative embodiments. An alternative embodiment may comprise two separate pulse generators having the quotient of their respective repetition rates equal to a certain ratio as above and each pulse generator 'being enabled by a signal whose time duration is to be monitored. The resultant pulse trains might be applied, respectively, to the advance and reverse inputs of a reversible counter. The counter, upon the attainment of a balance count condition, triggers a connected alarm circuit. A system of this design is capable of monitoring a particular ratio for an indefinite period of time. It has the disadvantage, however, of requiring far more sophisticated counter circuits.

The above embodiments are merely illustrative of many possible arrangements which can represent applications of the principles of the present invention. These and other arrangements can readily be devised in accordance with the principles of the present invention by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. The method of monitoring the critical ratio of the respective time durations of a first and a second independent signal during a specified time period comprising the steps of generating a iirst pulse train at a iirst preselected frequency, generating a second pulse train at a second preselected frequency, the ratio of the frequency of said first and second pulse trains being inversely proportional to said critical ratio, transmitting the pulses in said first pulse train to a counting facility in the presence of said first independent signal, transmitting the pulses in said second pulse train to said counting facility during the absence of said second signal, counting continuously with said counting facility the total number of pulses of both said rst and second pulse trains transmitted to said counting facility Within said specified time period, and detecting the attainment of a critical total count during said specified time period.

2. The method of monitoring the critical ratio of the respective time duration of a first and second signal as described in claim l further includingr the step of generating an auxiliary pulse Whenever the individual transmitted pulses of said first and second pulse trains are coincident and counting sai-d auxiliary pulses.

3. A critical ratio detector comprising, in combination, a first pulse generator having a predetermined pulse repetition rate, a second pulse generator having a predetermined pulse repetition rate, a first and a second signal, the critical ratio of the respective durations of said first and second signals being monitored for a predetermined measuring period, the ratio of the pulse repetition rates of said first and said second pulse generators being inversely proportional to said critical ratio, pulse counting means, enabling means to connect the output of said first pulse generator to said pulse counting means, said enabling means being responsive to said first signal, disabling means to disconnect the output of said second pulse generator -from said pulse counting means, said disabling means being responsive to said second signal, and alarm means responsive to a predetermined count in said counter.

4. A critical ratio detector comprising in combination, pulse generation means, a first source of gating signals responsive to the passage of a first series of objects past a first sensory device, said first gating signals having a first predetermined time duration, a second source of gating signals responsive to the passage of a second series of objects past a second sensory device, said second gating signals having a second predetermined time duration, pulse counting means, first gating means to transmit the pulse output of said pulse generation means to said pulse counting means in response to the presence of said first gating signals, second gating means to transmit the output of said pulse generation means to said pulse counting means in response to the absence of said second gating signals, said first and second predetermined time durations Ibeing inversely proportional to some critical ratio to be detected, and means to respond to a count indicative of the attainment of said critical ratio Nithin a specified time period.

5. A critical ratio detector comprising, in combination, a first pulse generator having a first predetermined pulse repetition rate, a second pulse generator having a second predetermined pulse repetition rate, a first and second signal whose critical ratio of respective durations is to be monitored, pulse counting means including means to advance and reduce the count in response to individual inputs, respectively, means to transmit the output of said rst pulse generator to said advance means in response to the presence of said first signal, means to transmit the output of said second pulse generator to said reducing means in response to the absence of said second signal, means to disable the transmission of the output of said second pulse generator to said reducing means in response to the presence of said second signal, and alarm means to respond to the attainment of a specified threshold count Within a specified time period to indicate the attainment of said critical ratio.

6. Apparatus to monitor the critical ratio of the respective time durations of a first and second signal comprising in combination a rst pulse source freely running at a first preselected pulse repetition rate, a second pulse source freely running at a second preselected pulse repetition rate, the quotient of said first and second pulse repetition rates being equal to the inverse of said critical ratio, said first and second signals having independent durations, pulse counting means, first gating means, means to apply said first signal to said first gating'means, said first gating means transmitting the pulse signals of said first pulse source to said pulse counting means in response to said first signal, second gating means, means to apply said second signal to said second gating means, said second gating means transmitting the pulse signals of said second pulse source to said counting means during the absence of said second signal and inhibiting the transmission of the pulse signals of said second pulse source in response to said second signal, and means coupled to said pulse counting means to detect a prescribed threshold level in said pulse counting means and means to generate an alarm signal in response thereto.

7. The apparatus in claim 6 further including means to generate a pulse in response to the coincidence of pulses from said first and second pulse sources, means to delay the transmission of said pulse for a period of time at least equal to the duration time of said coincident pulses, and means to apply said generated pulse to said pulse counting means.

References Cited UNiTED STATES PATENTS 9/1960 Gordon 340--171 X 5/1966 Buhler 340--168 X 

3. A CRITICAL RATIO DETECTOR COMPRISING, IN COMBINATION, A FIRST PULSE GENERATOR HAVING A PREDETERMINED PULSE REPETITION RATE, A SECOND PULSE GENERATOR HAVING A PREDETERMINED PULSE REPETITION RATE, A FIRST AND A SECOND SIGNAL, THE CRITICAL RATIO OF THE RESPECTIVE DURATIONS OF SAID FIRST AND SECOND SIGNALS BEING MONITORED FOR A PREDETERMINED MEASURING PERIOD, THE RATIO OF THE PULSE REPETITION RATES OF SAID FIRST AND SAID SECOND PULSE GENERATORS BEING INVERSELY PROPORTIONAL TO SAID CRITICAL RATIO, PULSE COUNTING MEANS, ENABLING MEANS TO CONNECT THE OUTPUT OF SAID FIRST PULSE GENERATOR TO SAID PULSE COUNTING MEANS, SAID ENABLING MEANS BEING RESPONSIVE TO SAID FIRST SIGNAL, DISABLING MEANS TO DISCONNECT THE OUTPUT OF SAID SECOND PULSE GENERATOR FROM 